1. Field of the Invention
This invention pertains generally to security guard devices, and is particularly concerned with devices of the type widely known in the trade as chain door fasteners.
2. Description of the Prior Art
Chain door fasteners of the prior art are conventionally comprised of a trim plate member adapted to be secured by screws to the jamb of a door frame, and a door plate member adapted aslo to being secured by screws to a door so as to be adjacent the trim plate member when the door is closed. A short length of chain, usually about 6 inches long, has one end swively coupled to the trim plate member, the other end of the chain being provided with a plug like element which is removably securable on the door plate member by means of a slideway slot defined by the latter member.
The plug element is received in an enlarged opening at one end of the slideway, and is thereupon slidable in the slot to the other end thereof whereat it is secure against removal from the door plate member. This arrangement provides a measure of security under circumstances where it is desired to open the door slightly (about 21/2 maximum) for ventilation or for checking the identity of a person on the other side of the door.
However, the security may be largely false in the case of an outsider determined to break in. If the door is already opened as aforesaid, it is a simple matter to insert a bolt cutter, or the like, through the narrow opening to sever the chain. In the event that cutting of the chain is frustrated, a determined intruder may likely effect entry by breaking in by impact force applied by his body to the side of the door adjacent the area of the chain door fastener. This is possible because the impact force jerks the chain out from the jamb. As a consequence of this the force is transmitted by the chain to the trim plate member in a direction which is substantially along the axis of the wood screws holding the plate on the jamb, and this is the weakest holding force direction of the usual wood screws employed.